The invention relates to a method and apparatus in unwinding. The invention relates to the splicing phase of a continuous unwind, in which the web of a new machine reel brought to the unwind is attached at full speed to the web of the machine reel that is becoming empty by cutting the web and pressing the web of the machine reel that is becoming empty against the splice in the new machine reel.
In off-machine coating machines for paper, a continuous unwind is used in which a new, full machine reel that is brought to the unwind is attached to the end of the paper web in the machine reel that is becoming empty. In present-day fast coating machines the splicing method is essentially the same, i.e. a splice is prepared in the leading end of the web of a new machine reel by means of a two-sided adhesive tape, the splice being attached on the surface of the reel by pieces of fastening tape. The surface speed of the new machine reel is accelerated to be equal to the running speed of the machine, i.e. the web speed of the paper web discharged from the reel, whereafter the web of the reel becoming empty is pressed against the aforementioned splice for example by means of a roll or a brush. The old web is cut with a blade above the splice.
The splicing in an unwind has become problematic at current running speeds (1200-1600 m/min). Therefore, the running speed of the coating machine often has to be dropped for the duration of the splicing. A negative pressure is generated in the so-called splicing gap between the splicing roll and the machine reel, which are brought close to each other at high speed, which negative pressure can be pulsating if the new machine reel is out-of-round. The negative pressure tends to draw the old web partly against the splice already before the splicing, and causes fluttering of the old web. Furthermore, the negative pressure tends to detach the tape splice from the surface of the new machine reel, wherein the new machine reel opens before splicing. In order to make the web travel in a controlled manner, a bend is necessary at the splicing roll, which, in turn, requires stretching of the web, when the splicing roll is rapidly struck against the surface of the machine reel. The aim is to keep the tension peak caused by the stroke of the roll on a low level, by using a small splicing gap (8 to 12 mm), which causes a strong phenomenon of negative pressure. Even the currently used bending angle causes a problematic tension peak in the web. At higher running speeds, an even larger bending angle would be necessary.
A solution to this problem is presented in the Finnish patent 100323, and in the related U.S. Pat. No. 5,709,355. The splicing device comprises a splicing roll, by means of which the the web of the machine reel that is becoming empty is pressed against the splice in the new machine reel, and at least one auxiliary roll. The splicing roll and the auxiliary roll are attached to a lever device, which is journalled at an articulation point between the axes of the rolls in such a way that before the splicing and in the splicing the web travels in such a manner that the length of the web is substantially the same both in the splicing position and in the basic position of said rolls.
The aforementioned solution has made it possible to eliminate the problems occurring in the vicinity of the splicing gap, and it enables a splicing with high running speeds of even over 1600 m/min. In the so-called flying splicing described above, the control of the cut tail of the old reel spool has become problematic at high running speeds and especially in connection with heavy paper grades. After the splicing, the aim is to rapidly stop the reel spool that has formed the core of the reel. The paper left on the reel spool must not be discharged therefrom to such an extent that it cannot be controlled by means of air blows. The discharged paper web as well as the pieces, so-called chaff, detached therefrom, tend to enter the nip between the splicing roll and the machine reel rotating in the primary station. This will almost always cause a break at the splicing device. Moreover, pieces detached from the web often travel along with the air currents on top of the web (e.g. from the sides of the machine), which also causes a break. By means of air jets it is not possible to fully control the paper travelling at high speed and the pieces detached therefrom.
The length of the paper discharged after cutting from the reel spool that is becoming empty is proportional to the gravity of the problem. In constructions currently in use, the aim is to minimize the length of the discharged paper with a shortest possible braking time. However, the braking time is increasing, because the speeds of the machines are growing, and as the sizes of the machine reels grow, the diameters of the reel spools grow as well. Even the present-day speeds and diameters of the reel spools set high demands on the braking devices, and the brakes have to be maintained and changed often, which increases the operating costs.
It is an aim of the invention to eliminate the aforementioned drawbacks and to present a method and apparatus, by means of which the problems caused by the splicing and the paper discharged after cutting can be avoided.
The invention is based on the idea that the end point of the dischargeable web on the reel is stored in a memory, which indicates the location of the end point in the longitudinal direction of the web, and when the web is discharged from the reel, the length of the discharged web is simultaneously monitored and compared to the memory. The comparison indicates the remaining amount of paper in linear measure units. When the end point of the web is approaching, it is possible to give a splicing command when a predetermined length is left on the reel. Thus, the rest of the length of the web on the reel that is becoming empty can be minimized, and this short web section which can still issue from the roll can be controlled more easily.
In practice it is possible to proceed in such a manner that for example in a rereeler before the coating machine after the tail threading, an attachment point for the web is produced on the bottom of the machine reel, for example a two-sided tape, or glue is fed thereto. The end point of the web can be attached either on the surface of the reel spool or on the paper layers on the bottom after a few windings, wherein the adhesive substances do not enter in contact with the surface of the roll.
Thereafter the person operating the machine performs the necessary steps by means of a storing device, for example a terminal. The attachment point is transferred to the memory as a position information indicating its location in the longitudinal direction of the machine.
Each machine reel is allotted a memory of its own, which can be read later under the control of a program in connection with unwinding. It is possible to use for example a fault map known as such as the memory. Thanks to an identification system for the machine reel, the reel is automatically accompanied with the machine reel specific fault map when passed to the unwinding for the paper coating machine. The edge of the paper is provided with locating marks at regular intervals, as well as beginning and end marks of the reel, by means of which the fault map is synchronized in the unwinding. In this system, the point in which the unwinding of the reel is proceeding is known with the accuracy of 1 to 2 m.
The invention is utilized to set the time of the splicing in the unwinding. The system gives advance information in the form of a signal when the end point of the web is approaching. On the basis of this information, the drive is capable of accelerating the full machine reel to the web speed at the correct time. Another signal gives a splicing command, whereafter the splicing sequence (striking the web onto the surface of the machine reel and cutting the web) is conducted when the full machine reel is in the correct position. The timing of the splicing command is set in such a way that the end point of the web, whereafter the web can no longer get loose from the reel, is brought as close to the surface as possible.